Iron- nickel alloy with low coefficient of thermal expansion for making shade masks

ABSTRACT

The invention concerns an alloy whereof the chemical composition comprises, by weight: 35%=Ni=37%. 0.001% %=C=0.05, % Mn=0.10%, Si=0.15%, Co=0.5%, S&lt;0.002%, P=0.006%, B=0.0005%, Al+Mo+Cu+Cr=0.15% 0.015%=2(V+Ti)+Nb+Zr+Ta+III=0.2%, 0.0025%=N+O=0.015% optionally calcium and/or magnesium with total content ranging beween 0.001 and 0.005%, the rest consisting of iron and unavoidable impurities resulting from preparation, and a method for making a strip of said alloy.

FIELD OF THE INVENTION

The present invention relates to an iron and nickel based alloy with a very low coefficient of expansion that can be used in particular for manufacturing shade or shadow masks for color display cathode ray tubes.

PRIOR ART

In order to avoid the local deformation through thermal expansion of the shadow masks used for color display cathode ray tubes, it is desirable for their manufacture to employ an alloy that has the lowest possible coefficient of thermal expansion. Thus, for example, use is made of an FeNi alloy containing about 36% nickel and about 0.3% manganese, well known by the name of Invar. Such an alloy has a coefficient of thermal expansion between 20° C. and 100° C. of the order of 1×10⁻⁶/K.

However, this coefficient of expansion is still too high for certain applications, such as application to flat screens, and it has been proposed that use be made of an FeNi alloy in which a few % of the nickel are replaced with cobalt. This alloy has the advantage of having a coefficient of thermal expansion of the order of 0.4×10⁻⁶/K, which leads to a saving of 60%, but has the disadvantage of containing cobalt. The problem with this is that shadow masks are metal sheets pierced with very fine holes obtained by chemical etching, and the cobalt leads to troublesome contamination of the chemical etching baths. Furthermore, cobalt is a very expensive element and it is desirable to reduce its content as far as possible.

Hence, it has been proposed that use be made of an FeNi alloy with a low residual and cobalt content, containing in particular under 0.1% manganese. This alloy has the advantage, on the one hand, of containing little or no cobalt and, on the other hand, of having a coefficient of thermal expansion of the order of 0.8×10⁻⁶/K, which is lower than that of the conventional FeNi alloy (Invar). However, the coefficient of expansion is still too high, particularly for large format or slimline flat screens.

Furthermore, it is desirable to use thinner masks so as to reduce their cost of manufacture and so as to improve the quality and precision of the images. Now, the mechanical properties of the alloys of the prior art are not good enough to allow the thickness of the masks to be reduced while at the same time keeping the masks to withstand the deformations that may arise during the various transport and handling steps.

SUMMARY OF THE INVENTION

The object of the present invention is to remedy the disadvantages of the alloys of the prior art by proposing an alloy that can be used in particular for the manufacture of shadow masks, that contains little or no cobalt, of which the coefficient of thermal expansion is lower than that of the known FeNi alloys, and which has an elastic limit in the annealed state that is maintained or even improved.

To this end, a first subject of the invention is an alloy, the chemical composition of which comprises, by weight: 35%≦Ni≦37% 0.001%≦C≦0.05% Mn≦0.10% Si≦0.15% Co≦0.5% S<0.002% P<0.006% B≦0.0005% Al+Mo+Cu+Cr≦0.15% 0.015%≦2(V+Ti)+Nb+Zr+Ta+Hf≦0.2% 0.0025%≦N+O≦0.015%

possibly calcium and/or magnesium in a total content of between 0.0001 and 0.005%,

the remainder consisting of iron and inevitable impurities resulting from the production process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment, the alloy furthermore exhibits a niobium content of below 0.1%, or even below 0.07%.

In another preferred embodiment, the alloy exhibits a carbon content of above 0.0035%.

In another preferred embodiment, the alloy exhibits a grain size below 10, or even below 9 (in accordance with G ASTM E 112).

In another preferred embodiment, the alloy has a coefficient of thermal expansion between 20° C. and 100° C. of below 0.70×10⁻⁶/K, and preferably of below 0.65×10⁻⁶/K. In any case, the coefficient of expansion obtained is below 0.75×10⁻⁶/K.

In another preferred embodiment, the alloy exhibits a conventional elastic limit at 0.2% OYS, in the annealed state, of above 250 MPa, preferably above 280 MPa, and as a more particular preference, above 300 MPa, or even 310 MPa.

In another preferred embodiment, the niobium and carbon contents of the alloy composition are such that: Nb×C≦0.01.

This embodiment allows the elastic limit of the grade in the annealed state to be improved through the formation of carbides on a sub-micron scale.

In another preferred embodiment, the titanium, niobium and nitrogen contents of the alloy composition are such that: Ti×N≦0.00006 Nb×N≦0.001.

This embodiment makes it possible to avoid the presence of excessive quantities of niobium and/or titanium nitrides, which have a size of the order of a few hundred nanometers or even a few microns, and which present problems when manufacturing the shadow masks by etching.

In another embodiment, the alloy contains precipitates based on titanium and/or on niobium and/or on vanadium and/or on tantalum and/or on zirconium and/or on hafnium, the mean size of which is equal to 100 nm or smaller, preferably equal to 70 nm or smaller and as a more particular preference, smaller than 50 nm.

A second subject of the invention is a method of manufacturing a strip of alloy, according to the invention comprising the steps whereby:

-   -   a semi-finished version of said alloy is hot-rolled after         reheating to a temperature of above 850° C. and below 1350° C.         so that the rolling temperature is above the solutionizing         temperature of the titanium- and/or niobium- and/or vanadium-         and/or zirconium- and/or tantalum- and/or hafnium-based         precipitates and so that the temperature at the end of rolling         is below the temperature at which said precipitates begin to         precipitate, so as to obtain a hot-rolled strip;     -   the hot-rolled strip is cold-rolled in one or more passes to         obtain a cold-rolled strip, possibly with one or more         intermediate annealing operations between two passes.

In a first preferred embodiment, the temperature of the intermediate annealing operation or operations performed during the cold-rolling is below the solutionizing temperature of said precipitates.

In a second preferred embodiment, the temperature of the intermediate annealing operation or operations performed during the cold-rolling is above the solutionizing temperature of said precipitates.

These two different embodiments allow the formation of precipitates and the grain size to be altered. By way of nonlimiting indication, a grain size larger than 7 is generally obtained for the first embodiment, whereas grain sizes smaller than 7.5 are generally obtained in the second embodiment.

In another preferred embodiment, the temperature at the end of hot-rolling is equal to 850° C. or lower, which makes it possible to obtain finer grains.

A third subject of the invention is the use of the alloy described hereinabove for the manufacture of shadow masks for color display cathode ray tubes, for the manufacture of shadow masks held in the vertical or horizontal direction for flat screen monitors, for the manufacture of shadow mask support frames, for the manufacture of cryogenic storage containers, and also for the manufacture of electron gun grids, by virtue of its very strong suitability for mechanical cutting.

The invention is based on the fact that the inventors discovered, in a novel way and surprisingly, that the precipitation of compounds formed from titanium and/or niobium and/or vanadium and/or zirconium and/or tantalum and/or hafnium, on the one hand, and from carbon, oxygen and/or nitrogen, on the other, leads to an appreciable reduction in the coefficient of expansion when the alloy has a low Si and Mn content. Precise analysis of the compounds formed is tricky, but in particular carbides, nitrides, carbonitrides, oxides and/or oxynitrides of the abovementioned metals are found.

Without wishing to be tied to a theory, the inventors believe that this effect could be due to the fact that these various compounds, for the most part, have a crystalline structure of the cubic type, and form precipitates the size of which is generally of the order of several tens of nanometers when they are formed in the solid phase. These small-sized precipitates precipitate in the matrix and not at the grain boundaries, as is conventionally the case.

This effect on the coefficient of expansion of the alloy is particularly visible in FIG. 1, which represents the variations of this coefficient between 20 and 100° C. as a function of the sum of the oxygen and nitrogen contents, for an alloy the composition of which contains titanium at contents ranging between 0.01 and 0.05%, less than 5 ppm of boron, less than 5 ppm of sulfur and no aluminum. The same effect is obtained with an alloy containing niobium completely or partially replacing the titanium, within the limits set by claim 1.

The alloy according to the invention contains, as % by weight:

-   -   from 35% to 37% nickel, and preferably between 35.5% and 36.5%,         so as to obtain a low coefficient of thermal expansion between         20° C. and 100° C.,     -   from 0.001% to 0.05% of carbon, so as to form fine carbide         precipitates. The formation of nanometer-scale carbide         precipitates has the effect of reducing the coefficient of         expansion and of improving the mechanical properties of the         product. Its content is limited to 0.05% so as to avoid the         formation of large insoluble carbide inclusions. It is         preferable for the carbon content to be above 0.0035% so as to         have enough of a carbon content by volume to obtain improved         mechanical characteristics. It is also preferable for the carbon         content to be kept to a value below 0.010%, or even below 0.007%         so as to further limit the size of the carbides formed.     -   under 0.1% manganese because this element increases the         coefficient of expansion of the alloy and needs to be limited,     -   under 0.15% silicon, because this element increases the         coefficient of expansion of the alloy and needs to be limited,     -   under 0.5% cobalt, so as not to contaminate the chemical etching         baths used to etch the shadow masks,     -   possibly 0.0001 to 0.005% of at least one element taken from         among calcium and magnesium so as to trap the sulfur which         always exists by way of an impurity and thus ensure good         hot-deformation ability,     -   possibly sulfur at a content of below 0.002% so as not to impair         the hot-conversion ability of the alloy,     -   possibly phosphorus in a content of below 0.006% so as not to         impair the hot-conversion ability of the alloy,     -   possibly boron at a content of less than 0.0005%, and preferably         of 0%: this is because the inventors have found that with boron         present, the coefficients of thermal expansion increase         appreciably,     -   possibly aluminum, molybdenum, copper or chromium in a total         content of less than 0.15%, because these elements increase the         coefficient of thermal expansion of the alloy,     -   titanium, vanadium, niobium, tantalum, zirconium and/or hafnium         in quantities such that the sum 2(V+Ti)+Nb+Ta+Zr+Hf lies between         0.015% and 0.2% so as to be able to form precipitates based on         these elements, these precipitates preferably exhibiting a mean         size smaller than 100 nm, and as a preference smaller than 70         nm, and as a particular preference, smaller than 50 nm. It is         furthermore preferable for the niobium content to be below 0.1%,         or even below 0.07%, so as to further reduce the coefficient of         expansion and the size of the precipitates,     -   oxygen and/or nitrogen in quantities such that the sum of their         contents lies between 0.0025% and 0.015%, because the inventors         have found, in a novel way, that the presence of oxygen and/or         of nitrogen in these contents in the alloy allows the         coefficient of expansion to be lowered when it is associated         with the presence of titanium and/or niobium and/or vanadium         and/or tantalum and/or zirconium and/or hafnium. The sum of         these contents is limited to 0.015% so as to avoid the formation         of large oxides or nitrides,     -   the remainder of the composition is made up of iron and of         impurities resulting from the production process.

The alloy may be formulated, for example, in an arc furnace with an AOD or VOD converter refining phase; it may also be formulated in an induction furnace under vacuum. This formulation must be carried out in such a way as to obtain the desired residual contents.

The alloy is then cast into the form of a semi-finished product such as an ingot, a billet or a remelting electrode. It may also be cast directly in the form of a thin slab or thin strip less than 15 mm thick, and preferably with a thickness of between 8 and 12 mm.

When the alloy is cast in the form of a remelting electrode, this electrode is remelted under electrically-conducting slag so as to obtain better homogeneity of the chemical composition and of the solidification structure.

The semi-finished product or the thin strip obtained by direct casting is then hot-rolled at a temperature of above 850° C., and preferable above 1150° C. but below 1350° C. to obtain a hot-rolled strip with a thickness of generally between 2 mm and 6 mm, and preferably between 3 and 5 mm, which is then cold-rolled in one or more passes, possibly with annealing operations above 800° C. The temperature to which the strip is heated between the hot-rolling or cold-rolling steps may be chosen in such a way that the precipitates of oxides, carbides, or nitrides may possibly be returned to solution. Rapid coolings may also be applied in order to keep these elements likely to form precipitates in solid solution within the alloy. Equilibrium precipitation treatments may then be carried out by temperature soaks at between 750° C. and 1200° C. (but preferably below 1050° C.).

The invention will now be described in greater detail but without implying limitation, and illustrated by examples.

Tests

By way of example, the alloys identified 1 to 16 according to the invention and 17 to 23 by way of comparison and the composition of which is described in Table 1 below were produced. The chemical compositions and the coefficients of expansion a between 20 and 100° C. were measured on test specimens taken from the hot-rolled strips. Each of these test specimens was annealed for 30 minutes at 950° C., and cooled in ambient air before the coefficient of thermal expansion measurements were taken. The results of the tests are collated in Table 2, in which the coefficient of expansion α is expressed in 10⁻⁶/K.

The etching tests were performed on cold-rolled products from the experimental castings, partially coated with photosensitive resin. The etchings were performed at 60° C. with an FeCl₃ solution having a density of 45.5°Bé. The quality of the etching was evaluated by measuring the regularity of the cut contours, and through the presence of defects associated with the presence of particles. TABLE 1 No. Ni Mn Si Al Co C S N O Nb V Ti B Examples 1 35.80 0.048 <0.007 0.009 0.011 0.003 0.0010 0.0036 0.0019 <0.005 <0.005 0.023 <0.0005 according 2 35.84 0.044 <0.007 <0.005 0.010 0.003 0.0010 0.0016 0.0024 <0.005 <0.005 0.017 <0.0005 to the 3 36.08 0.027 0.021 <0.005 0.010 0.002 <0.0005 0.0023 0.0041 <0.005 <0.005 0.012 <0.0005 invention 4 36.13 0.027 0.011 <0.005 0.009 0.003 <0.0005 0.0020 0.0016 <0.005 <0.005 0.034 <0.0005 5 36.08 0.029 0.053 <0.005 0.011 0.003 0.0005 0.0030 0.0024 <0.005 <0.005 0.024 <0.0005 6 36.16 0.030 0.078 <0.005 0.010 0.003 0.0005 0.0031 0.0012 <0.005 <0.005 0.048 <0.0005 7 36.09 0.031 0.020 0.044 0.009 0.003 <0.0005 0.0026 0.0013 <0.005 <0.005 0.022 <0.0005 8 36.06 0.030 0.021 0.055 0.010 0.002 <0.0005 0.0028 0.0010 <0.005 <0.005 0.052 <0.0005 9 36.10 0.040 0.045 0.008 0.050 0.004 0.0009 0.0023 0.0018 0.030 <0.005 0.016 <0.0005 10 36.10 0.045 0.040 <0.005 0.048 0.004 0.0008 0.0030 0.0015 <0.005 0.020 0.010 <0.0005 11 36.15 0.040 0.030 <0.005 0.050 0.004 0.0008 0.0032 0.0017 0.040 <0.005 0.005 <0.0005 12 36.20 0.042 0.033 <0.005 0.035 0.003 0.0009 0.0030 0.0015 0.028 <0.005 0.015 <0.0005 13 36.15 0.041 0.032 <0.005 0.050 0.003 0.0010 0.0026 0.0017 0.035 <0.005 0.009 <0.0005 14 36.18 0.051 0.027 0.008 0.014 0.004 0.0009 0.0021 0.0012 0.060 <0.005 0.015 <0.0005 15 36.0 0.06 0.03 <0.005 0.28 0.0044 0.0007 0.0031 0.0012 0.051 <0.005 <0.005 <0.0005 16 36.1 0.03 0.025 0.006 0.05 0.0048 0.0005 0.0025 0.0015 0.055 <0.005 <0.005 <0.0005 Comparative 17 35.84 0.052 <0.007 0.013 <0.005 0.003 0.0008 0.0042 <0.001 <0.005 <0.005 0.013 0.0010 Examples 18 35.83 0.053 0.011 0.019 0.011 0.006 0.0006 0.0034 0.0012 <0.005 <0.005 0.025 0.0024 19 35.79 0.049 <0.007 0.038 0.012 0.002 0.0028 0.0021 <0.001 <0.005 <0.005 0.045 <0.0005 20 36.00 0.071 0.076 <0.005 0.049 0.005 0.0007 0.0025 0.0012 <0.005 <0.005 <0.005 <0.0005 21 35.95 0.042 0.021 <0.005 0.068 0.002 0.0029 0.0013 0.0012 0.051 <0.005 <0.005 <0.0005 22 35.80 0.039 <0.007 0.006 <0.005 0.002 0.0005 0.0010 0.0013 0.009 0.012 0.080 <0.0005 23 36.2 0.045 0.041 <0.005 0.050 0.002 0.0008 0.0003 <0.001 0.040 <0.005 0.007 <0.0005

TABLE 2 Mean Conventional coefficient elastic of thermal Grain limit at expansion size*³ 0.2% OYS between 20 Quality of (annealed No. (MPa)*¹ and 100° C. the etching*² state) 1 300 0.53 P 8 2 297 0.57 G 8 3 307 0.52 G 8.5 4 300 0.52 P 8 5 298 0.56 P 8 6 292 0.61 P 7.5 7 301 0.62 P 8.5 8 291 0.59 P 7.5 9 332 0.57 G 9.5 10 327 0.54 G 9 11 320 0.49 G 8.5 12 328 0.56 G 9.5 13 322 0.50 G 9 14 325 0.61 G 9 15 285 0.54 G 6.5 16 289 0.51 G 6.5 17 289 0.80 G 8.5 18 298 1.01 P 8.5 19 295 0.76 P 8.5 20 275 0.75 G 8 21 302 0.75 G 8.5 22 290 0.77 P 8.5 23 313 0.78 G 9 *¹Values measured after a heat treatment for 15 minutes at 850° C. *²G: etching deemed to be good - P: presence of faults associated with the presence of particles. *³Grain size measured in accordance with G ASTM E 112, to plus or minus 0.5 of a unit.

In the light of this table, all the strips according to the invention can be seen to have a coefficient of expansion of below 0.70×10⁻⁶/K and even of below 0.65×10⁻⁶/K in most cases.

By contrast, the strips given by way of comparison have coefficients of expansion appreciably higher than 0.70×10⁻⁶/K.

The comparative examples 17 and 18 show the detrimental effect of boron on the coefficient of expansion. The comparative examples 19 and 21 show the detrimental influence of sulfur on the coefficient of expansion. These comparative examples also show the importance that the oxygen and nitrogen contents have on the coefficient of expansion.

The comparative example 20 which corresponds to the conventional low-manganese FeNi alloy provides the reference that demonstrates the advantages of the invention. Specifically, in the absence of compounds allowing solid phase precipitates to form, the coefficients of expansion measured are higher.

Comparative example 21 shows the detrimental effect of sulfur on the coefficient of expansion.

Comparative examples 22 and 23 show the importance of the nitrogen and oxygen contents on the coefficient of expansion.

The alloy according to the invention can also be used for the manufacture of shadow mask support frames. This alloy has good behavior during chemical etching and this is associated with the controlled weak presence of residuals of the C,S,N type in solid solution, and because of its small amounts of micron-scale inclusions. 

1. An alloy, the chemical composition of which comprises, by weight: 35%≦Ni≦37% 0.001%≦C≦0.05% Mn≦0.10% Si≦0.15% Co≦0.5% S<0.002% P<0.006% B≦0.0005% Al+Mo+Cu+Cr≦0.15% 0.015%≦2(V+Ti)+Nb+Zr+Ta+Hf≦0.2% 0.0025%≦N+O≦0.015% possibly calcium and/or magnesium in a total content of between 0.0001 and 0.005%, the remainder consisting of iron and inevitable impurities resulting from the production process.
 2. The alloy as claimed in claim 1, which furthermore has a niobium content of below 0.1%.
 3. The alloy as claimed in claim 1 or 2, which furthermore has a carbon content of above 0.0035%.
 4. The alloy as claimed in any one of claims 1 to 3, and the grain size of which is below 10 (in accordance with G ASTM E112).
 5. The alloy as claimed in any one of claims 1 to 4, and which exhibits a coefficient of thermal expansion between 20° C. and 100° C. of below 0.7×10⁻⁶/K.
 6. The alloy as claimed in any one of claims 1 to 5, and of which the conventional elastic limit at 0.2% OYS in the annealed state is above 280 MPa.
 7. The alloy as claimed in claim 6, and of which the conventional elastic limit at 0.2% OYS in the annealed state is furthermore above 300 MPa.
 8. The alloy as claimed in any one of claims 1 to 7, wherein the niobium and carbon contents are furthermore such that: Nb×C≦0.01.
 9. The alloy as claimed in any one of claims 1 to 8, wherein the titanium, niobium and nitrogen contents of the alloy composition are furthermore such that: Ti×N≦0.00006 Nb×N≦0.001.
 10. The alloy as claimed in any one of claims 1 to 9, and which contains precipitates based on titanium and/or on niobium and/or on vanadium and/or on tantalum and/or on zirconium and/or on hafnium, the mean size of which is equal to 100 nm or smaller.
 11. A method of manufacturing a strip of alloy as claimed in any one of claims 1 to 10, comprising the steps whereby: a semi-finished version of said alloy is hot-rolled after reheating to a temperature of above 850° C. and below 1350° C. so that the rolling temperature is above the solutionizing temperature of the titanium- and/or niobium- and/or vanadium- and/or zirconium- and/or tantalum- and/or hafnium-based precipitates and so that the temperature at the end of rolling is below the temperature at which said precipitates begin to precipitate, so as to obtain a hot-rolled strip, the hot-rolled strip is cold-rolled in one or more passes to obtain a cold-rolled strip, possibly with one or more intermediate annealing operations between two passes.
 12. The method as claimed in claim 11, wherein the temperature of the intermediate annealing operation or operations performed during the cold-rolling is below the solutionizing temperature of said precipitates.
 13. The method as claimed in claim 11, wherein the temperature of the intermediate annealing operation or operations performed during the cold-rolling is above the solutionizing temperature of said precipitates.
 14. The method as claimed in claim 11 or 12, wherein the temperature at the end of hot-rolling is equal to 850° C. or lower.
 15. The use of an alloy as claimed in any one of claims 1 to 10 for the manufacture of shadow masks for color display cathode ray tubes.
 16. The use of an alloy as claimed in any one of claims 1 to 10 for the manufacture of cryogenic storage containers.
 17. The use of an alloy as claimed in any one of claims 1 to 10 for the manufacture of electron gun grids.
 18. The use of an alloy as claimed in any one of claims 1 to 10 for the manufacture of shadow masks held in the vertical or horizontal direction for flat screen monitors.
 19. The use of an alloy as claimed in any one of claims 1 to 10 for the manufacture of shadow mask support frames. 